Gemfile.lock: update net-ldap to fix LDAP authentication issues

Newer LDAP servers fail with older versions of net-ldap.
Update to the fixed version.

Signed-off-by: David Aguilar <davvid@gmail.com>
Merge pull request #5 from davvid/net-ldap

Gemfile.lock: update net-ldap to fix LDAP authentication issues